1. Field of the Invention
The present invention relates to a laser irradiation apparatus to crystallize the semiconductor film and the like or to activate them after ion implantation by using laser light. In addition to that, the laser irradiation apparatus according to the present invention includes a laser irradiation apparatus to irradiate the laser light on the semiconductor film which is polycrystalline or near-polycrystalline, and improve (promote) crystallinity of the semiconductor film. Furthermore, the present invention relates to a method of manufacturing a semiconductor device using the crystalline semiconductor film formed by the laser irradiation apparatus above.
2. Description of the Related Art
In recent years, the technology to form TFT on a substrate makes great progress and application development to an active matrix type semiconductor device is advanced. Especially the TFT with the polycrystalline semiconductor film is superior in field-effect mobility to TFT with a conventional amorphous semiconductor film and thereby high-speed operation becomes possible. Therefore, it has been tried that pixel that was controlled by the driver circuit provided outside of the substrate so far is controlled by the driver circuit formed on the same substrate as the pixel.
By the way, a substrate for the semiconductor device represented by the TFT is expected to be a glass substrate rather than a monocrystal silicon substrate in terms of its cost. However, a glass substrate is inferior in heat resistance and easy to change its shape when heated. Therefore, when forming the TFT with a polysilicon semiconductor film on a glass substrate, in order not to change the shape of the glass substrate because of heat, laser annealing is performed to crystallize the semiconductor film.
The characteristic of laser annealing is that the processing time can be drastically shortened when compared with annealing method by radiation heating or conductive heating, and that a semiconductor substrate or a semiconductor film can be heated selectively and locally so that the substrate will be hardly damaged thermally.
It is noted that the laser annealing method described here indicates the technology to recrystallize an amorphous layer or a layer damaged by the impurity doping formed on the semiconductor substrate or the semiconductor film, or the technology to crystallize an amorphous semiconductor film formed on the substrate. Moreover, the technology to planarize or modify the surface of the semiconductor substrate or the semiconductor film is also included.
The lasers used for laser annealing are classified broadly into two types according to its oscillation system. In recent years, it has been known that in crystallization of the semiconductor film, a crystal grain formed in the semiconductor film is larger when using a continuous oscillation laser than when using a pulse oscillation laser. When the crystal grain formed in the semiconductor film is large, the number of the grain boundary included in the TFT channel region formed by using the semiconductor film decreases and thereby the mobility becomes high. As a result, such semiconductor film can be applied to a device with high-performance. For this reason, the continuous oscillation laser is beginning to attract attention.
Moreover, when performing laser annealing on the semiconductor or the semiconductor film, the method to convert a laser beam emitted from the laser by an optical system so as to become an elliptical shaped or a line shaped beam and scan a beam spot (surface to be irradiated by the laser) to a surface to be irradiated is known. This method enables an effective irradiation of the laser light on the substrate so that mass-productivity can be enhanced and is superior in the industrial purpose. Therefore this method is employed preferably. (Reference: patent document 1 for example)
Patent Document 1: Japanese Patent Application Laid-Open Hei. 8-195357
In order to perform laser annealing on the semiconductor film formed on the substrate effectively, the method to convert the shape of the laser light emitted from the continuous oscillation laser into the line shape or the elliptical shape by an optical system, and scan the converted beam to the substrate is employed.
In addition, a galvanometer mirror is used as a means to scan the laser light. That is, the laser light which is incident into the galvanometer mirror is deflected to the direction of the substrate and by oscillating the galvanometer mirror to control the incident angle and reflecting angle of the laser light to the galvanometer mirror, the deflected laser beam can be scanned to the whole surface of the substrate. With the structure that the laser light can be scanned only by oscillating the galvanometer mirror, it is not necessary any more to move the substrate back and forth by a stage and the like, and thereby it becomes possible to perform laser irradiation in a short period of time.
It becomes possible to focus the beam deflected by the galvanometer mirror constantly on the plane surface by converging with an fθ lens. The beam deflected by the galvanometer mirror is scanned from the edge to the center of the lens and thereby the beam is scanned on the substrate arranged on the plane surface, that is, the semiconductor film.
However, the transmissivity of the fθ lens used as a means converging laser light is different in the center and the edge thereof. Therefore, when the fθ lens is used as it is for crystallization by a laser, energy distribution of the laser light irradiated on the semiconductor film is not uniform and thereby the laser light cannot be irradiated uniformly on the whole semiconductor film. When irradiating the laser light on the semiconductor film, however, the semiconductor film needed to be processed uniformly by irradiating the laser light uniformly.
Therefore, an object of the present invention is to provide the laser irradiation apparatus of continuous oscillation that can perform a laser irradiation effectively and uniformly. That is to say, the present invention provides the means to offset the difference in the energy distribution due to the difference of the transmissivity of the above lens and homogenize the irradiation energy of the laser light on the surface to be irradiated.